In recent years, multiband compatible radiophone apparatuses which can utilize a plurality of frequency bands have been developed. In these multiband compatible radiophone apparatuses and wireless communication systems utilizing a plurality of frequency bands, when the radiophone apparatus moves, it becomes necessary to switching from a base station communicated with at present to a base station using a different frequency band.
The function of switching base stations (communication channels) communicated with is called “hand-off” (or “hand-over”). Hand-off includes two types: soft hand-off and hard hand-off (see, for example, Patent Document 1).
“Soft hand-off” is hand-off performed without switching frequencies. The base station communicated with at present (handing-off channel) and the base station desired to be newly communicated with (handed-off channel) are temporarily set in a state of simultaneous communication, then the processing for switching the base stations is carried out.
As described above, in soft hand-off, the radiophone apparatus is always communicating with one or more base stations, therefore communication is not interrupted at the time of hand-off.
However, soft hand-off is not always possible (soft hand-off cannot be carried out in a case where two base stations cannot provide services to a radiophone terminal by the same frequency), therefore hard hand-off is carried out.
“Hard hand-off” maintains communication with a first base station up to just before switching communication to a second base station when changing from the first base station to the second base station, then switches the frequency band. For this reason, the channel in use ends up being temporarily released, so communication ends up being interrupted transiently when switching frequency bands.
Patent Document 1: Japanese Patent Publication (A) No. 2003-244742